The invention relates to a process for co-production of paraxylene and metaxylene from a feedstock that contains aromatic hydrocarbons with 8 carbon atoms.
The production of paraxylene with high purity by separation by adsorption in a simulated moving bed is well known from the prior art. This market is extensively developed: its outlets are the production of terephthalic acid, phthalic anhydride and polyethylene terephthalate resins. In contrast, the metaxylene market is still restricted, whereby its outlet is isophthalic acid. It was recently perceived that the addition of small amounts of polyethylene isophthalate to polyethylene terephthalate improved the properties of the latter. It therefore becomes advantageous to co-produce paraxylene and metaxylene in the same aromatic compound production complex provided that the market requirements are satisfied: the amount of paraxylene that is produced should be much larger than that of metaxylene: typically 5 to 40 times larger, the paraxylene should be very pure, typically at least 99.6%, and the metaxylene should have reasonable purity, typically at least 99.0%.
The prior art knows metaxylene production processes, for example U.S. Pat. No. 4,326,092 where the adsorbent is a Y zeolite of an Si/Al molar ratio on the order of 4.5 to 5 exchanged with sodium and where the separation is carried out by the technique of adsorption in a simulated moving bed in liquid phase. In U.S. Pat. No. 5,382,747, the same separation is carried out on a Y zeolite that is exchanged with lithium and sodium, in a restricted range of temperature and degree of hydration by using the toluene as a desorbent. To co-produce the paraxylene in the great majority and metaxylene, the drawback of these processes is to require two separate units of very different sizes, without a possibility of finding any synergy in the co-production of the two isomers.
The prior art also describes processes of co-production of paraxylene and metaxylene; for example, U.S. Pat. No. 4,368,347 uses a vapor phase process with intermediate fraction recycling: in addition to the complication that is linked to recycling of intermediate fractions, this document does not suggest how it is possible to use in a practical way such a process that operates at a pressure of between 1 and 2 bar and at a temperature of 150 to 200xc2x0 C. with a feedstock whose bubble point is 145xc2x0 C. and with fixed beds that have pressure drops of at least 0.1 bar and probably more to operate economically. Patent FR 2 651 148 uses two different solvents to separate the C8-aromatic fraction into three effluents, which greatly limits its scope since the distillations that result from the simulated moving bed separation unit are multiplied. Patent WO 93/22022 describes various cases of separations of feedstocks of three components into three effluents, however the technology that is used that involves very high pressures, pressure regulation and flow rate regulation at the same time in each of the three or four zones of the process and beds that are each separated in a column is justified economically only for products of high added value.
U.S. Pat. No. 4,306,107 describes a simulated moving bed process in liquid phase where the metaxylene is sampled in the form of extract; the paraxylene, orthoxylene and a fraction of ethylbenzene are sampled as an intermediate raffinate; and finally the ethylbenzene is sampled as a raffinate. This process naturally does not allow a majority of paraxylene and an accompanying stream of metaxylene to be co-produced.
The document of the prior art that comes closest to the invention is U.S. Pat. No. 4,313,015; this document describes the separation of a simulated moving bed, in liquid phase, on zeolite X that is exchanged with barium, whereby the desorbent is diethylbenzene. The extract consists of paraxylene that is too impure (99.44%) to be marketed at current standards (current standard=99.6 mini) and with a yield of 97.5%; the intermediate raffinate consists of ethylbenzene, metaxylene and orthoxylene and a little paraxylene; finally the raffinate consists primarily of a mixture of orthoxylene and metaxylene, whereby the metaxylene can be separated by distillation. The text specifies that the intermediate raffinate is sampled approximately in the middle of the zone between the introduction point of the feedstock and the sampling point of the raffinate. The feedstock that is dealt with in the example is not completely representative of a feedstock that is found in a refinery: the latter always contain at least traces and sometimes up to 5% of paraffins and naphthenes with eight and nine carbon atoms, which distill in the same temperature interval as xylenes. This document does not specify how the paraffins and naphthenes are separated between the raffinate and the intermediate raffinate, the total number of beds and the number of beds per zone used, the order of magnitude of the flow rates in each of the zones of the process, and more particularly the one in zone 1, therefore the necessary solvent level, and the switching time of the beds (circulation speed of the solid).
One of the joint inventors of this application with two other inventors filed a patent application now publication FR 2782714 in which a process for continuous co-production of paraxylene and metaxylene from a hydrocarbon feedstock in at least one chromatographic column that contains at least twenty-five beds that are distributed in five zones is described. At least five beds should be located in zone 3B that is between the point for continuous draw-off of an intermediate raffinate that contains metaxylene, orthoxylene, ethylbenzene, solvent and paraxylene, and the point for continuous draw-off of a raffinate that contains metaxylene and orthoxylene and solvent. In addition to the large number of beds necessary for the implementation of the process (30, for example), the hydrocarbon feedstock should have an ethylbenzene content that is less than 5%, which is restricting. Furthermore, despite these constraints, although the purity of the metaxylene that is obtained after distillation under economical conditions of said raffinate is very advantageous (for example 99.6%), the yield of metaxylene relative to the metaxylene that is contained in the feedstock of the adsorption unit is about 10%, which is low.
The object of the invention is therefore the co-production of paraxylene with a purity of at least 99.6% and with a minimum yield of 98%, and metaxylene with a purity after distillation that is at least equal to 99% and with a yield that is greater than 10%, according to the definition above, and preferably greater than 15%, for example between 15 and 20%.
Another object is to produce in particular metaxylene from a feedstock that is not limited in terms of ethylbenzene.
More specifically, the invention relates to a process for co-production of metaxylene and paraxylene from a hydrocarbon feedstock that comprises them, whereby the process comprises a separation stage of said mixture in a simulated moving bed in at least one chromatographic column that contains a number of beds of an adsorbent that are interconnected in a loop, whereby said column comprises an injection of a feedstock, a draw-off of a first raffinate, a draw-off of a second raffinate that comprises metaxylene, an injection of desorbent and a draw-off of an extract that delivers paraxylene, whereby the process comprises the simultaneous periodic offsetting of injection positions and the draw-off position of the extract from a bed in the direction of flow of the main flux that circulates in the column, whereby the process is characterized in that a first raffinate that comprises desorbent, orthoxylene, metaxylene and ethylbenzene is drawn off continuously or intermittently, in that a second raffinate R2 that comprises orthoxylene and metaxylene is drawn off intermittently, whereby the process is also characterized in that the second raffinate is distilled in such a way as to recover, separately, orthoxylene and metaxylene with at least 99% of purity, preferably with at least 99.5%, and with an improved yield.
The advantages of the process according to the invention relative to that of the prior art are as follows:
It is possible to work with a smaller number of beds in the chromatographic column, for example with a number of beds that is at least equal to 20.
With a number of beds that is equivalent to that of the prior art where the metaxylene is produced continuously and with an isopurity of metaxylene in the second raffinate, a better yield of metaxylene is obtained in the second raffinate relative to the initial feedstock. This yield may be multiplied by a factor of 2 relative to the one that is carried out in continuous production.
According to a characteristic of the process where the first raffinate can be drawn off continuously or intermittently, it is possible to move forward during simultaneous periodic offsetting the injection positions and the draw-off position of the extract, the position of the draw-off of the first raffinate of a bed or of two beds in the direction of the flow of the main flux.
According to a first implementation of the process where the second raffinate is drawn off intermittently, it is possible to move forward during simultaneous periodic offsetting the injection positions and the draw-off position of the extract, the draw-off position of the second raffinate of at least two beds or at most a submultiple of the total number of beds, and the second raffinate is drawn off during one period.
According to a first variant, during one period it is possible to draw off the first raffinate intermittently, the pressure of the chromatographic column is controlled at the level of the feedstock, or the desorbent or the extract, preferably at the level of the feedstock, and the first raffinate and the second raffinate are drawn off under flow monitoring.
According to this variant, the process can be implemented according to several embodiments:
Axe2x80x94According to a first embodiment, it is possible to draw off during a period Tn, from a given bed n, the second raffinate plus the first raffinate during next period Tn+1, from the same bed n, no raffinate is drawn off from next bed n+1, the second raffinate of next bed n+2 is drawn off during the period Tn+2, then the first raffinate during next period Tn+3 from same bed n+2, and so on.
Bxe2x80x94According to a second embodiment, it is possible to carry out the following sequence multiple times: in a given bed n, the second raffinate is drawn off during a period Tn, in same bed n, the first raffinate is drawn off during next period Tn+1, in next bed n+1, the first raffinate is drawn off during next period Tn+2, and no raffinate is drawn off in next bed n+2.
Cxe2x80x94According to a third embodiment, it is possible to carry out the following sequence multiple times: in a given bed n, the first raffinate is drawn off during a period Tn; in next bed n+1, the first raffinate is drawn off during a period Tn+1; in next bed n+2, the first raffinate is drawn off during a period Tn+2; no raffinate is drawn off in next bed n+3 during next period Tn+3, and a second raffinate is drawn off in next bed n+4 during same period Tn+3.
According to a second variant, to draw off the first raffinate continuously, it is possible to draw off from one bed the first raffinate for at least one period, and under pressure monitoring and during another period, a minor portion of the first raffinate is drawn off from another bed at a minimum flow rate that is compatible with pressure monitoring, and a major portion of the second raffinate is drawn off simultaneously under flow monitoring.
According to a second implementation of the process where the second raffinate is drawn off intermittently, it is possible to move forward during simultaneous periodic offsetting the injection positions and the draw-off position of the extract, the position of draw-off of the second raffinate from at least one bed or at most a submultiple of the total number of beds, and the second raffinate is drawn off during a fraction of the period.
More specifically, during a fraction of the period, it is possible to draw off from a bed a minor portion of the first raffinate under pressure monitoring and a major portion of the second raffinate under flow monitoring, then during the remaining portion of the period, only the first raffinate is drawn off from the same bed under pressure monitoring, and the sequence above is repeated in the next bed in the next period and so on.
The process according to the invention can be implemented in liquid phase at a temperature that is generally between 20xc2x0 C. and 260xc2x0 C., preferably between 90xc2x0 C. and 210xc2x0 C., and under a pressure of between atmospheric pressure and 18 bar (1 bar=0.1 MPa).